“Stripped” magnetic particles. Applications of the double surfactant layer principle in the preparation of water-based magnetic fluids

Abstract

The double surfactant layer principle for colloidal stability is applied to the design of magnetite particles having primary and secondary surfactant layers composed of different fatty acids. Concentrated aqueous dispersions, containing up to 32% by weight Fe 3O 4, are prepared, which are stable to dilution (60 fold). Specific magnetizations of up to 28 J T −1 kg −1 (28 emu/g) are recorded. Particles are initially stripped of surfactant which is surplus to the primary layer, in a treatment prior to the addition of the secondary surfactant layer. The particles having solely a primary surfactant layer, disperse readily in nonpolar solvents to give fluids containing up to 33% by weight Fe 3O 4. Differences occur for some fatty acids between the use of NH + 4 and CH 3NH + 3 salts for the formation of the secondary surfactant layer, and are ascribed to the ready decomposition of NH 4 salts at 90°C. Oleic acid and Sarkosyl-“O,” having unsaturation in their hydrocarbon chains, are found to be suitable for the primary surfactant layer when straight chain fatty acids are used as the second surfactant, but not for use as the secondary surfactant layer. Redispersion of double surfactant layer coated particles, separated from aqueous media at pH < 5, has been achieved using (CH 3) 4N +OH −. As the pH rises dispersion first occurs, followed by particle separation, then redispersion occurs again at high pH. The last stage is attributed to charge stabilization. The double surfactant layer design of the surfactant sheath is achieved also using partially esterified polyacrylic acid, but not for polyacrylic acid itself.